Transistor radio, perhaps? Effectively, you have the channel through which the FM information is transmitted. Your radio picks up, via the antenna, all of that information. There is a mechanism through which you tune a filter that selects only the desired frequencies and all others are blocked. Amplification ensues and it drives your speakers. With regards to the frequency domain, all signals can be characterized by their frequency components, just like time domain signals are characterized by their time characteristics. You simply exploit this by designing circuitry to block unwanted frequencies.
A radio signal of 98.3MHz would simply have a frequency domain characteristic of a single peak at 98.3MHz. All this tells you is that all of the power in that signal is contained at that frequency. Here, for example, is a 1KHz signal represented in the frequency domain.
Sampling the environment around you might yield something like this (although the numbers are not likely to match up exactly, since FM radio is transmitted at higher frequencies than those shown here):
As you can see, there are several peaks representing power in the transmission channel. Your radio receives all of this information and filters out the unwanted data. This can happen either in digital space, through software, or in the analog space, through electrical components. The implementation for each are quite different, but the overall goal is the same - filtration.
Ideally, you would want to turn something like this into the something more like the first image. Although, FM transmission, as its name implies (Frequency Modulation), is a bit more complex than encoding information using a single frequency.
To create a spectrogram, a signal (e.g audio) is split into segments and then the FFT of each of those segments is computed and then the individual amplitude spectrums of those segments are put side by side.That is why with a spectogram you have both time and frequency components.You might want to see the diagram in THIS post so you can visualise how a spectrogram is produced.
Best Answer
The concept is good, but you will find it is not so simple in practice.
Pitch is not simply the predominant tone, so there's problem number 1.
The FFT frequency bins can't hit all (or even multiple) tones of the musical scale simultaneously.
I would suggest playing with an audio program (for example, Audacity) that includes an FFT analyser and tone generator to get a feel for what it can (and can't) do before you try to implement a particular task using the FFT.
If you need to detect just a few specific tones, you may find the Goertzel algorithm to be easier and faster.
Pitch detection is complicated, and there is still research going on in that field. Tone detection is pretty straight forward, but may not get you what you want.